A prescription writing anatomy :)

℞:  medication details
Disp.:  dispensing instructions (subscription)
Sig.: patient instructions

- Prescription: The word "prescription", from "pre-" ("before") and "script" ("writing, written"), refers to the fact that the prescription is an order that must be written down before a compound drug can be prepared. (per wiki).

- Super-scription (Rx = meaning recipe, sig.):  "The symbol "℞" separates the superscription from the inscriptions sections. In this arrangement of the prescription, the "℞" is a symbol for recipe or literally the imperative "take." This is an exhortation to the pharmacist by the medical practitioner, "I want the patient to have the following medication" – in other words, "take the following components and compound this medication for the patient." (per wiki). Ex: the entire prescription

- Inscription (= med): name, strength, dose of the med. Ex: 25mg Diphenhydramine 12.5mg/5mL 

- Sub-scription (dispensing instructions for pharmacist): This may be compounding instructions or quantities (per wiki). Ex: #'s of pills

- Sig. (signa, signature): pt instruction. Ex: Take 2 pills at bedtime.
Helpful Sources:

• Prescription format (helpful info.): https://en.wikipedia.org/wiki/Medical_prescription#Format_and_definition
• Practice writing prescriptions (writing exercise with answer keys): http://www.d.umn.edu/medweb/Modules/Prescription/Practice.html
• Preventing Prescription Errors (with rich illustrations): https://www.health.ny.gov/publications/1418.pdf 
• Recommendations to enhance accuracy of prescription/medication order writing: http://www.nccmerp.org/recommendations-enhance-accuracy-prescription-writing
• Prescription writing to maximize patient safety: http://www.aafp.org/fpm/2002/0700/p27.html
• Prescription writing manual: http://apps.who.int/medicinedocs/en/d/Jwhozip23e/5.4.html